This invention relates to a reclamation of lead-sulphuric acid storage batteries, and particularly to the treatment of liquid wastes from the reclamation process prior the disposal.
The waste material from storage battery reclamation contains large amounts of sulphuric acid along with heavy metals including large quantities of iron and smaller but still significant quantities of highly toxic metals such as lead, antimony, arsenic and copper. Because of the acidity and the presence of toxic metals, the waste must be treated prior to disposal.
Conventional treatment methods involve neutralization, followed by sedimentation for removal of suspended solids generated in neutralization and filtration. Ammonia and caustic soda are commonly used for neutralization, but because of the high solubility of the neutralization products, the salinity of the body of water receiving the discharge of the treatment process is adversely affected. Lime is also used as a neutralizing agent. Where lime is used, a great deal of calcium sulphate precipitates. However, calcium sulphate is partially soluble in water, and the resultant of the neutralization is a saturated solution of about 2000 ppm. of calcium sulphate which can also have a detrimental effect. Also in conventional treatment, precipitation and filtration do not remove heavy metals adequately, and therefore toxic quantities of heavy metal ions may remain in the discharge of the treatment process.
The main object of this invention is to provide a process for treating battery wastes which produces a discharge which is extremely low in dissolved solids and which contains negligible quantities of toxic metals.
The process includes neutralization and precipitation, and utilizes reverse osmosis in the discharge stage to insure the removal of heavy metals and dissolved calcium sulphate. As is well-known, osmosis is the process by which a liquid on one side of a semi-permeable membrane passes through the membrane into a solution on the other side, thereby reducing the concentration of the latter. The osmosis process is reversible by the application of pressure to the solution side of the membrane. In reverse osmosis, a liquid containing dissolved matter is applied under high pressure to one side of a semi-permeable membrane. Essentially pure water is forced through the membrane, and the concentration of the solution on the high-pressure side of the membrane is increased. Unfortunately, in the case of lime-neutralized battery waste, the solution is already saturated with calcium sulphate, and therefore reverse osmosis would normally result in the precipitation of solids which would foul the semi-permeable membranes. In accordance with the invention, the calcium sulphate is sequestered by the addition of a sequestering agent, preferably sodium hexametaphosphate. The addition of the sequestering agent permits reverse osmosis to take place satisfactorily, without precipitation of calcium sulphate.
In accordance with a preferred embodiment of the invention, the solution containing sequestered calcium sulphate is returned to the precipitation stage. Lime is added to the solution and causes the hexametaphosphate to revert to orthophosphate. Calcium phosphates and calcium sulphate then precipitate and are removed in the precipitation stage. Preferably the addition of lime takes place by the use of the concentrated solution from the reverse osmosis as a diluent for the neutralizing lime.
The discharge of the system is essentially pure water, passed through the reverse osmosis membrane, containing less than 100 ppm. dissolved solids and less than 1 ppm. of heavy metal salts. Precipitated calcium sulphate, calcium phosphate, and heavy metal hydroxides are removed by clarification and filtration, and dissolved solids such as sodium salts, leave the system with the water which accompanies the precipitated matter.